Dig deeper to better manage soil acidity
Is soil testing the topsoil (0-10cm) the best way to manage soil acidity?
Southern Farming Systems (SFS) is investigating soil acidity and crop and pasture
responses to lime with projects supported by GRDC, Corangamite Catchment
Management Authority the Australian Federal Government.
Project Coordinator Lisa Miller says sampling of 130 paddocks in south-west Victoria has provided
good insights into how soil pH can change with depth and how far lime is moving.
“Growers generally sample soils in the top 10cm. We tested 0-10, 10-20 and 20-30cm samples to get
a better representation of the soil profile,” she said.
SFS found pH was fairly similar at 0-10cm and 10-20cm but there was a big discrepancy when lime
had been put on in the last three to five years. Soil pH was much greater in 0-10cm layer reflecting
that liming had worked but the soil pH in the 10-20 cm soil layer remained unchanged and was still
very acidic.
Lisa thought there were two possible reasons. Either all the lime was effectively being used up
neutralising the acid in the topsoil and none was left over to move beyond 10 cm or that lime was
taking more than three years to move beyond 10 cm.
“We found surface applied lime had moved 6 cm in 6 months after application so we suspect that we
are using up all the lime within the topsoil. This creates a potential problem as the underlying soil
layers will keep acidifying and increase in toxic aluminium which roots will not grow into,” she said.
Lisa recommends monitoring the 10-20 cm layer to check lime is moving down the profile. If it’s not
moving then the question becomes what can growers do about it?
Research done in WA and NSW found that you need to keep soil pH(Ca) above 5.5 in the topsoil to
get surface applied lime to move beyond 10 cm and stop subsoil acidity from occurring.
“This is a much higher pH than what we feel comfortable with, so we will be testing to see if this is
the case. We are also considering trying to incorporate lime at depth,” she said.
SFS also found soil pH generally increased with depth and in the 20-30cm soil layer. However 8% of
mixed cropping farms were an exception with pH decreasing and below the target of 4.8. For more
information visit www.sfs.org.au
Photo: Jim Caldwell, SFS soil testing at the SFS Inverleigh Trial Site
By Lisa Miller, SFS Research & Extension Officer
For Stock and Land publication 6 August 2015